Study the effects of 85 specified compounds on DNA repair in mammalian cells. Determine by density gradient centrifugation of DNA obtained from mammalian cells treated with carcinogens whether the uptake of labeled thymidine into nonreplicating DNA is an accurate reflection of DNA repair. Determine whether human lymphocytes may serve as well as rodent fibroblasts in the detection of ten to twelve carcinogens. Determine, using appropriate enzymatic metabolic activating systems, whether procarcinogens, non-carcinogens, and ultimate carcinogens can stimulate the incorporation of labeled thymidine into non-replicating DNA. All 85 compounds will be used in this study both with and without an enzymatic activating system.